Warm Glass
MOLD MANUFACTURING AND PREPARATION
The traditional mold material used for kiln casting is a 50/50 mixture of plaster and ground silica. Molds for slumping can also be made using a ratio of two parts plaster to one part silica. Generally, a mixture of 200-mesh silica and gypsum plaster, both of which are available from pottery suppliers, are used.
Mold may be made using materials other than plaster and silica. In some formulations, the basic mixture is augmented with binders or other substances. Commercial preparations, such as Mold Mix 6, are also available. If you decide to use one of these alternative formulations, follow the manufacturer's recommendations for preparation and mixing.
The Traditional Plaster/Silica Mixture
There are two main criteria for a casting mold. First, it should be strong enough to hold the melted glass. Second, it must be resistant to the heat of the kiln. Many mold materials are also resistant to glass sticking, so no kiln wash or other barrier is usually needed.
-- Preparing a box for the mold.
Once the model is completed, you will need to build a box to surround it to hold the mold material until it dries. The box should be about one to two inches larger than the model on all sides. It can be made from cardboard, linoleum scraps, or a similar inexpensive substance.
The box should be affixed to a firm surface such as a sheet of glass or cardboard base. Hot glue works well for this purpose. You will also need to affix the model to the bottom of the mold box before covering it with your investment material.
-- Mixing the investment.
The plaster/silica mixture, often called an "investment," should be mixed in a clean container. Using a container that has not been thoroughly cleaned may contaminate the mixture or cause it to set up more quickly than desired.
There are many different investment formulas, but the simplest uses equal parts of silica flour (around 200 mesh), pottery plaster, and water. These ingredients can be weighed out prior to mixing, or (if such precision goes against your nature) you can mix by feel, adding as you go. With a bit of practice, you'll be able to estimate the correct amount of mixture you need to make.
It's best to finish the mold in one pouring, but if you mix too little you can let the mold set up and add more once it dries. Don't be discouraged if your first few attempts don't come out exactly right; with a little practice you'll soon become an expert.
Mix the two dry ingredients together in a separate container. You should wear a respirator or approved mask when working with silica to keep from inhaling the potentially harmful silica dust.
Put the water in the mixing container. Use clean, room temperature tap water. Some people believe the results are better if you allow the water to sit for a few hours before using.
Once the investment is mixed, begin adding it to the water. (Always add the dry ingredients to the water, not the water to the dry plaster and silica.) Add the dry mixture a little at a time, stirring by hand between additions. Some people use a sifter to evenly spread the dry mixture on the surface of the water; others simply sprinkle across the surface.
Continue adding the investment until it "peaks." Peaking occurs when the mixture sinks slowly and dry investment islands appear in the container. (If you measured out your ingredients properly, the mixture should "peak" just as you use the last of the dry ingredients.)
Once the investment peaks, give the mixture one final stir to mix in all the dry ingredients. Then stop mixing and allow the mixture to sit undisturbed for about five minutes. This process, called "slaking," helps ensure that the investment particles become saturated with water.
-- Pouring the mold.
After allowing the investment to slake for five minutes, slowly pour it into the box that surrounds your model. To minimize bubbles and distortion of the model, pour the investment along the edges of the box, rather than directly on the model. Do not pour leftover mixture down the sink, as it will harden and ruin your plumbing.
-- Removing the model.
After the mold has air-dried (this may take several days), you should be able to remove the model. Clay models can generally be worked loose with a stick or removed by hand. They can be difficult to remove from deep inside crevices, so are best used for more open molds.
Wooden or paper models can be burned out, although it may be necessary to remove ashes and any other possible contaminants from the mold after firing.
Wax models will need to be melted out. Once this has been done, the remaining cavity can be filled with frit or cullet and then fired. This process, which is adapted from the jewelry-making field, is called "lost wax casting." Click here to read more about the basics of the lost wax casting process.
-- Drying the mold.
Air drying the mold is a necessary step, but it is generally not sufficient to totally dry (or "cure") the mold. You also need to heat the mold in a kiln to remove any lingering moisture or contaminants from your model.
The key to kiln drying is to take it slow. The slower you heat, the stronger the mold will be. Soak the mold at 225 degrees F and again at 350 degrees F. This is to drive off any remaining water and help stabilize the mold. Continue to heat until the temperature reaches about 1200 F. In general, the slower you heat the better. Allow the mold to cool slowly back to room temperature, then it's ready to use.
Cracks sometimes appear in the mold after curing; if these are small they won't present a problem, but large cracks may require repair or even starting over again. These cracks are more likely to occur if you fired too quickly or if the mold is large.
One interesting aspect of casting with molds is that you are far more likely to crack the molds than you are the glass. This is contrary to slumping, in which the glass is more at risk. In casting, the mold tends to protect the glass, but its tendency toward cracking requires that heating and cooling take place more slowly than if you were simply fusing or slumping glass.
Click here to go to the next part of the Kiln Casting section, "Filling the Mold with Glass."